Test for issue with Fallbacks and enforce_available_locales

Created a test-case for an issue where the
Fallbacks module will generate an
I18n::InvalidLocale error.

Fallbacks relies on I18n.translation to find out
whether there's a translation for a fallback
locale (e.g. :'de' for :'de-DE'). But
I18n.translation does not distinguish from a
"user" call and one coming from the Fallbacks
module, and may raise I18n::InvalidLocale if
enforce_available_locales is set.
Fixes issue with Fallbacks and enforce_available_locales

Fallbacks should check whether each fallback
location being tried was set by
store_translations before deferring to
I18n.translate. Otherwise a I18n::InvalidLocale
may be raised when enforce_available_locales is
set.